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A dual-axis optical encoder and autocollimator with near-perfect orthogonality, stability, and high accuracy
NASA Goddard Space Flight Center invites companies to license its new absolute Cartesian technologies. The absolute Cartesian optical encoder relies on a combination of mature technologies to measure the X-Y planar motion of an object in a true Cartesian coordinate frame with high accuracy and near-perfect orthogonality. Originally developed to support the Hubble Space Telescope program, the absolute Cartesian encoder can be used for inspection, manufacturing, and assembly.
Relying on the orthogonal two-dimensional technology provided by the absolute Cartesian encoder, Goddard’s Cartesian electronic absolute autocollimator solves the instability problems associated with conventional autocollimators. The technology provides higher sensitivity in the same package or similar sensitivity in a much smaller package than conventional methods. It also offers a larger field of regardmaking it ideal for high-accuracy metrology and aerospace applications.
The absolute Cartesian encoder has been deployed in several applications at NASA, including optical metrology, radiometric calibration, and cryogenic environments. The technology is also valuable for the following applications:
How It Works NASA Goddard’s design uses a backlit, microlithographically patterned scale that is attached to a moving object. The scale carries X-Y information that uniquely identifies the horizontal and vertical location of the scale image as seen by a fixed electronic image sensor, thereby allowing determination of the absolute Cartesian position of the object. Why It Is Better Traditional X-Y positioning systems with closed-loop feedback encoders consist of two, usually stacked, linear translation stages. Because two encoders must be deployed to encode X and Y accurately, cost and complexity are increased, limiting viable applications. Linear encoders offer accurate coordinate determination but problems with this approach include imperfect alignment of the encoder to the linear directions of travel, the inability to account for lack of straightness of travel of the mechanical axes, and the lack of orthogonality of the directions of travel. Meanwhile, laser interferometers with plane mirrors offer high accuracy and resolution but are very expensive. Unlike these other encoder options, NASA Goddard’s design uses a microlithographically patterned absolute encoder scale containing all of the information necessary to encode any position within the range of X-Y travel. Because the information on the scale is strictly Cartesian, many problems associated with other encoding methods are eliminated. In addition, because one encoder serves both axes, cost and complexity are reduced. Nonstraightness of travel and lack of orthogonal mounting of stages are accounted for, and accuracy is limited only by that of the scale lithography. NASA has demonstrated a 15-cm x 15-cm (6-inch x 6-inch) absolute Cartesian encoder with coarse feature separation. Resolution of around 50 nm has been measured. Travel is not limited to that size. Even greater resolution, below 10 nm (competitive with many laser interferometers), is obtained using a more finely patterned scale, which still does not challenge the state-of-the-art in microlithography. Measurement speed or conversion rate is a function of the image sensor frame rate and the image processing software. NASA has demonstrated a 20-Hz measurement rate using a very inexpensive CCD-based camera. Using more advanced modern digital image sensors in conjunction with inexpensive digital signal processors (DSPs) has improved measurement speed to well in excess of 1 kHz.
Used to support development of the James Webb Space Telescope’s Near Infrared Camera (NIRCam) as well as for other NASA infrared space instruments, the Cartesian electronic absolute autocollimator is also valuable for use in the following applications:
How it works NASA Goddard’s technology replaces the conventional target with a Cartesian encoder scale, consisting of coded X and Y gridlines. An area array image sensor replaces the typical PSD, and analog signal-processing circuitry is usurped by computational image processing. A captured image is surveyed to find row and column indices of the intersections of the gridlines, and binary coded identities are derived by examining the code bits for each gridline. The two-dimensional angular relationship of the flat mirror is computed by determining how far off the center row and column of the image sensor each gridline is with respect to an arbitrary reference angle with extreme sensitivity. Why it is better NASA Goddard’s design eradicates many of the problems faced by conventional electronic autocollimators. Notably, traditional designs suffer from readout instability due to drifts in their analog circuit elements and to temperature effects on the PSD. This readout circuitry includes several stages of amplifiers for raw signal gain, X and Y signal sums and differences, and ratios of amplified sum and difference signals in X and Y. Even in best-case-scenario conditions, each of the five to seven amplifiers in the chain will suffer its own inherent gain and bias drifts. NASA Goddard’s autocollimator processes digital images from its area array image sensor rather than from a conglomeration of drift-prone analog amplifiers. Therefore, the technology provides highly stable linear output that does not change due to the temperature of the detector or its processing electronics. NASA has demonstrated stable performance of its full-size autocollimator (350-mm focal length) at an impressive 0.08 arc seconds peak-to-peak and 0.01 arc seconds root-mean-square (rms). Higher angular sensitivity compared with conventional designs is also achieved because the absolute Cartesian encoder technology enables centroid location determination to a finer dimensional scale than allowed by the traditional PSD. Further, absolute encoding of each individual gridline in the target and image ensures unambiguous position readout, and the orthogonality of the target gridlines assure the highest degree of orthogonality of azimuth and elevation readouts over the field of view. The field of view itself is also larger than that of a conventional autocollimator because the design projects a multiplet of gridlines, making the edges of the Cartesian scale visible for a greater distance to the right and left of the field of view. NASA has demonstrated fields of view between 0.42 and 2.3 degrees for its autocollimators, compared with a smaller 0.33-degree field of view for a commercially available autocollimator.
NASA Goddard has patented the absolute Cartesian encoder technology (U.S. Patent No. 6,765,195) and is seeking patent protection the absolute Cartesian autocollimator technology. (Link opens new browser window.)
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These technologies are part of NASA’s Innovative Partnerships Program Office, which seeks to transfer technology into and out of NASA to benefit the space program and U.S. industry. NASA invites companies to consider licensing the absolute Cartesian encoder (GSC-14330-1) technology and/or the Cartesian electronic absolute autocollimator (GSC-14718-1) technology for commercial applications. For information and forms related to the technology licensing and partnering process, please visit the Licensing and Partnering page. (Link opens new browser window) If you are interested in more information or want to pursue transfer of this technology, please contact: Innovative Partnerships Program Office |
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